Finite element modelling of development of stresses in thermal barrier coatings

Abstract

Thermal barrier coatings (TBCs) are used to allow higher gas temperatures (and hence greater efficiencies) in power generation gas turbines and/or to lengthen blade lifetimes, by reducing the heat transfer from the combustion gases to the blade substrate materials. However, the lives of TBC coated components tend to be limited by the growth of an oxide layer between the thermally insulating top coat and the MCrAlY coated superalloy substrate; this results in stresses which can lead to spallation (flaking-off) of the top coat. The present paper gives an overview of a recent programme of modelling work undertaken to understand the development of stresses due to the growth of the oxide layer. Typical examples of the rough interface between top coat and bond coat are characterised in terms of their aspect ratios. Representative geometries are then studied using a series of 2D finite element models of the interface layer. Initial models assumed a simple parabolic growth law for the oxide layer; the models were then developed to consider the evolving properties of the substrate and bond coat, and a more rigorous model of the oxidation process was implemented. The resulting model takes as its input the results of a microstructure evolution model developed at Loughborough University, which provides phase proportions. These in turn are used in conjunction with a constitutive model based upon an analytical homogenisation (based on Eshelby approach) that allows the substrate and bond coat creep and elastic behaviour to be predicted as the microstructure evolves. The formation of the thermally grown oxide (TGO) is modelled by considering the volume change due to oxidation. In turn, the model predicts the evolution of stresses at positions within the TGO layer. The influences of interface roughness, temperature and bond coat formulations are all explored by running the coupled model with different input parameters.